Embodiments of the invention relate generally to graphene fibers and, more particularly, to graphene fibers comprising intercalated large-sized graphene oxide (LGGO)/graphene sheets and small-sized graphene oxide (SMGO)/graphene sheets having high thermal and electrical conductivities and high mechanical strength.
Single-layer graphene has the highest thermal conductivity ever reported (up to 5,000 Wm−1K−1 at room temperature), Young's modulus (˜1,100 GPa), fracture strength (130 GPa), and mobility of charge carriers (200,000 cm2V−1s−1). These properties, however, are on a molecular level and have not been achievable when incorporated into graphene fibers.
Macroscopic graphene oxide (GO) fibers can be assembled from a dispersion of GO in aqueous media, with graphene fibers produced upon reduction of the GO fibers. The anisotropic liquid crystalline behavior of the GO sheets can lead to a pre-aligned orientation which can further be directed under shear flow to form an ordered assembly in a macroscopic fiber structure via a simple and cost-effective wet spinning process. Improvement of the mechanical properties of the GO fibers and graphene fibers can be achieved by introducing metal ion cross-linking bonds between graphene/GO sheets or by forming graphene/GO-based composite fibers (e.g., by adding carbon nanotubes).
GO fibers are typically electrically insulating. Electrical conductivity can be recovered on the order of 104 S/cm upon thermal or chemical reduction and can be further increased to about 9.3×104 S/m through doping with silver nanowires. The reported mechanical and electrical properties of graphene fibers, however, are orders of magnitude lower than those of single-layer graphene and are significantly inferior to commercialized carbon fibers and carbon nanotube fibers.
To date, it has been difficult to simultaneously achieve high mechanical and superior thermal and/or electrical conductivity properties in graphene fibers. Highly aligned sp2 graphene sheets are required for high thermal or electrical transport, in which case the mechanical strength is primarily due to van der Waals interaction between graphene sheets. At the same time, heterogeneous structures, including functional groups and sp3 bonds in cross-linked graphene nanosheets necessary to improve mechanical strength act as phonon and electron scattering centers, reducing electrical and thermal conductivities.